Reducing Effect of Magnetic and Electromagnetic Fields on an Implant&#39;s Magnet and/or Electronics

ABSTRACT

An implantable magnet that can freely turn in response to an external magnetic field, thus avoiding torque and demagnetization on the implantable magnet. The implantable magnet can be combined with an electric switching function depending on the orientation of an external magnetic field, thus protecting an implanted coil and/or implant electronics against induction of over-voltage or performing an electric switching function for other various purposes. The magnetic switch may further include, for example, a first switching contact and a second switching contact. A magnetically soft body that includes an electrically conductive surface is shiftable between a first position where the body is in simultaneous contact with the first and second switching contacts, and a second position where the body is out of contact with at least one of the first and second switching contacts. The body and the implantable magnet are positioned such that the body is shifted to one of the first position and the second position as a function of the external magnetic field resulting in a magnetic force between the magnet and the magnetically soft body.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/475,705, filed Jun. 27, 2006, which in turn is a continuation of U.S.patent application Ser. No. 10/994,950, filed Nov. 22, 2004, whichissued as U.S. Pat. No. 7,091,806, which in turn is a divisional of U.S.patent application Ser. No. 10/405,093, filed Apr. 1, 2003, which issuedas U.S. Pat. No. 6,838,963, which in turn claims the benefit of U.S.Provisional Patent Application No. 60/369,208, filed Apr. 1, 2002, andU.S. Provisional Patent Application 60/387,455, filed Jun. 10, 2002.Each of these references is incorporated herein by reference, in theirentirety.

TECHNICAL FIELD

The present invention generally relates to a magnetic switch, and amethod and apparatus for reducing the effect of magnetic fields andelectromagnetic fields on an implanted magnet and electronics in animplant, respectively.

BACKGROUND ART

Partly implantable systems may use magnets to hold internal and externalpieces in place. For example, as shown in FIG. 1, a cochlear implant 102located under the skin 103 and embedded in bone 104 typically include afirst magnet 106 placed in the center of the implant 102, and a coil108. An external part 101 includes a second magnet 105 that ispositioned over the first magnet 106 such that the external part 101 isheld against the implant 102 in an optimum position. By maintaining sucha position, an external coil 107 positioned inside external part 101can, via inductive coupling, transmit a transcutaneous signal and/orpower to the coil 108 of implant 102.

Upon a wearer of such a cochlear implant 102 having to undergo MagneticResonance Imaging (MRI) examination, interactions between the implantedmagnet 106 and the applied external MRI magnetic field may, at higherfield strength (i.e. above about 1 Tesla), produce two harmful effects.First, as shown in FIG. 2, the implanted magnet 202 may experience atorque that can twist the magnet 202 and the implant 201 out ofposition, thereby injuring the implant wearer, as shown in FIG. 2.Secondly, due to the external magnetic field, the implanted magnet maybecome partly demagnetized and may not be strong enough after the MRIfield exposure to hold the external part in place.

Another potentially dangerous effect may occur when RF pulses emitted bythe MRI unit induce voltages in the implant coil, implant circuit and/orelectrode circuit. These voltages may generate unwanted stimulation,especially in implants with analog electronic circuitry. Additionally,over-voltages may be generated which could destroy the implantelectronics.

Still other adverse effects can occur when a patient with a cochlearimplant undergoes an MRI examination. For example, artifacts may appearin the MRI image. These artifacts are caused by the local magnetic fieldof the implanted magnet, which distorts the homogeneous MRI field.

Present efforts to address the above-described problems includegenerally forbidding a patient with the cochlear implant to undergo thehigh-field MRI examination. However, this may exclude the patient fromcertain important diagnostic measures. Alternatively, the implant can bedesigned to minimize certain risks related to MRI examinations. Forexample, to avoid risks related to the implant magnet, the implant couldbe designed in such a way that the magnet can be removed from thepatient before MRI examination and be reinserted afterwards. Thisrequires two surgical interventions in order to perform a MRIexamination, and makes it impossible to use the implant system duringthe healing phase of the incision. Furthermore, the necessity tosurgically remove the magnet before the MRI examination is a drawbackespecially in emergency cases and even could be forgotten in some cases.Other ways to minimize the risk of a torque exerted on the implantmagnet is to use two identical implant magnets with oppositeorientation, as described in U.S. Pat. No. 6,348,070 issued to Teissland Hochmair, or to use a magnetically soft material (also called a“keeper”) instead of a permanent magnet inside the implant. At present,the magnetically soft material used for the keeper has been limited tosolid alloys or Ferrite.

Current methodologies to reduce the risks related to induction ofpossibly excessive voltages in the implant coil and circuits during anMRI examination include, for example, adding a Zener diode or a similarelectronic component to the electronic circuit. Other designs includethe use of two implant coils with opposite direction so as to reduce theinduced voltages in the implant coils, and the use of REED contacts asdescribed in U.S. Pat. No. 6,348,070 issued to Teissl and Hochmair.

SUMMARY OF THE INVENTION

In a first embodiment of the invention there is provided a magneticswitch. The magnetic switch includes a first switching contact and asecond switching contact, and a magnet free to rotate such that themagnet is capable of aligning at least partially with an externalmagnetic field. A magnetically soft body that includes an electricallyconductive surface is shiftable between a first position where the bodyis in simultaneous contact with the first and second switching contacts,and a second position where the body is out of contact with at least oneof the first and second switching contacts. The body and magnet arepositioned such that the body is shifted to one of the first positionand the second position as a function of the external magnetic fieldresulting in a magnetic force between the magnet and the magneticallysoft body.

In related embodiments of the invention, the magnet may, withoutlimitation, be a sphere or cylindrical. The magnetically soft body mayhave a calotte having a curvature that follows an adjacent surface ofthe magnet. The switch may be enclosed in a housing, which may benon-magnetic. The switch may be hermetically encapsulated. The switchmay be included in a cochlear implant, the implant further including afirst coil adapted to be electromagnetically coupled with a second coilof an external device. The switch may provide overvoltage protection forthe cochlear implant, as a function of an orientation of the externalmagnetic field.

In accordance with another embodiment of the invention, an implantincludes a housing. At least one magnet is free to turn in the housingsuch that the at least one magnet is capable of aligning at leastpartially with an external magnetic field.

In related embodiments of the invention, the at least one magnet isadapted to be magnetically attracted to a non-implanted magnet of anexternal device so as to hold the external device in a desired positionrelative to the implant. The implant may be a cochlear implant. Theimplant may include a first coil adapted to be electromagneticallycoupled with a second coil of an external device. The housing may be anencapsulation that hermetically seals the at least one magnet. Thehousing may include a magnetically soft material at least partiallysurrounding the at least one magnet. The housing may be made oftitanium, nonmagnetic stainless steel, or a ceramic. The adjacentsurfaces of the housing or magnet may be lubricated, or be made ofTeflon. The at least one magnet may be spherical or cylindrical in shapeand/or coated with a conductive surface. The at least one magnet may bea plurality of spherically shaped magnets.

In accordance with another embodiment of the invention, an implantincludes a magnet adapted to be magnetically attracted to anon-implanted magnet of an external device so as to hold the externaldevice in a desired position relative to the implant. A magneticallysoft material at least partially surrounds the magnet so as to reducethe torque and/or demagnetization of the magnet caused by an externalmagnetic field. The implant may be a cochlear implant.

In accordance with another embodiment of the invention, a magneticswitch includes a housing and a first and second switching contactpositioned within the housing. Additionally, a magnet is positionedwithin the housing. The magnet is free to rotate in the housing andincludes at least one conductive surface covering a portion of themagnet. The magnet is capable of rotating, in response to an externalmagnetic field, to one of a first position and a second position. In thefirst position, the at least one conducting surface is in contact withboth the first and second switching contacts. In the second position,the conducting surface is out of contact with at least one of the firstand second switching contacts.

In related embodiments of the invention, the conducting surface covers amagnetic pole. The magnet may be substantially spherical. The magnet mayinclude a recessed region, which may be non-conductive. The switch maybe part of a cochlear implant. The cochlear implant may include a firstcoil adapted to be electromagnetically coupled with a second coil of anexternal device. The switch may provide overvoltage protection for thecochlear implant, as a function of an orientation of the externalmagnetic field.

In accordance with another embodiment of the invention, a magneticswitch includes a housing and at least one spring contact. Each contactincludes a pin for driving the contact. A magnet is positioned in thehousing and is free to rotate in the housing such that it is capable ofaligning at least partially with an external magnetic field. The magnetfurther includes at least one recessed and/or grooved region. The magnetand the pin are positioned such that the magnet contacts and drive thepin when the pin is not aligned with one of the at least one recessedregions. The pin is not driven when the pin is aligned with one of theat least one recessed regions.

In related embodiments of the invention, at least one recessed region ispositioned at a magnetic pole. The magnet may include a north pole and asouth pole, the at least one recessed region positioned between thenorth pole and the south pole. The switch may be part of a cochlearimplant, and may include a first coil adapted to be electromagneticallycoupled with a second coil of an external device. The switch mayprovides overvoltage protection as a function of an orientation of theexternal magnetic field.

In accordance with yet another embodiment of the invention, an implantincludes a housing. A magnet is positioned within the housing, themagnet free to rotate such that the magnet is capable of aligning atleast partially with an external magnetic field. A magnetically softbody is coupled to the housing. The magnetically soft body is positionedadjacent the magnet so as to provide a path of low reluctance to themagnet.

In related embodiments of the invention, the magnet is one of aspherical magnet and a cylindrical magnet. The body may include acalotte shaped to fit the surface of the magnet. A lubricant may beplaced between the magnet and the body. The magnet may include a Teflonsurface. The magnet may be adapted to be magnetically attracted to anon-implanted magnet of an external device, so as to hold the externaldevice in a desired position relative to the implant and such that themagnetically soft body is positioned between the magnet and thenon-implanted magnet.

In accordance with another embodiment of the invention, an implantincludes a housing. A volume of Ferrofluid is located within thehousing, wherein the Ferrofluid becomes magnetized in the direction ofan external magnetic field.

In related embodiments of the invention, the volume of Ferrofluid isadapted to be magnetically attracted to a non-implanted magnet of anexternal device, so as to hold the external device in a desired positionrelative to the implant. The implant may be a cochlear implant.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying drawings, in which:

FIG. 1 is a graphical illustration of a cochlear implant with anexternal part held against the cochlear implant (PRIOR ART);

FIG. 2 is a graphical illustration of an implanted magnet of a cochlearimplant experiencing a torque as a result of an external magnetic field(PRIOR ART);

FIG. 3 is a graphical illustration of an implanted magnet encapsulatedin a spherical shell, in accordance with one embodiment of theinvention;

FIG. 4( a) is a graphical illustration of a top view of an implant thatincludes three free rotating spherical magnets, in accordance with oneembodiment of the invention;

FIG. 4( b) is a graphical illustration of a side view of the implant ofFIG. 4( a), in accordance with one embodiment of the invention;

FIG. 5( a) is a graphical illustration of an implanted magnet embeddedin magnetically soft matter, in accordance with one embodiment of theinvention;

FIG. 5( b) shows the implant of FIG. 5( a) with an external part heldagainst the implant, in accordance with one embodiment of the invention;

FIG. 6( a) is a graphical illustration of implant that includesFerrofluid with an external part held against the implant, in accordancewith one embodiment of the invention;

FIG. 6( b) shows the implant of 6(a) when immersed in an externalmagnetic field of a MR scanner, in accordance with one embodiment of theinvention;

FIG. 7 is a graphical illustration of an implant that includes aspherical magnet and a calotte made of soft magnetic material, inaccordance with one embodiment of the invention;

FIG. 8( a) is a graphical illustration of an implant that includes aspherical magnet, a calotte made of soft magnetic material, and aswitching function, in accordance with one embodiment of the invention,with no external magnetic field;

FIG. 8( b) shows the implant of FIG. 8( a) with an external magneticfield perpendicular to the implant's rotational symmetry axis, inaccordance with one embodiment of the invention;

FIG. 8( c) shows the implant of FIG. 8( a) with an external magneticfield parallel to the implant's rotational symmetry, in accordance withone embodiment of the invention;

FIG. 9 is a graphical illustration of an implant that includes acylindrical magnet and a switching function, in accordance with oneembodiment of the invention;

FIG. 10 is a graphical illustration of an implant that includes aspherical magnet and a switching function, in accordance with oneembodiment of the invention;

FIG. 11 is a graphical illustration of a top view of an implant thatincludes a spherical magnet and switching function, in accordance withone embodiment of the invention;

FIG. 12( a) is a graphical illustration of a side view of the implant ofFIG. 11 with an external magnetic field orientated perpendicular to theimplant's rotational symmetry axis, in accordance with one embodiment ofthe invention;

FIG. 12( b) is a graphical illustration of a side view of the implant ofFIG. 11 with an external magnetic field orientated parallel to theimplant's rotational symmetry axis, in accordance with one embodiment ofthe invention;

FIG. 13 is a graphical illustration of an implant that includes aspherical magnet and a switching function, the magnet having a recess,in accordance with one embodiment of the invention;

FIG. 14( a) is a graphical illustration of an implant that includes aspherical magnet and a switching function, the magnet having a recess,with an external magnetic field orientated parallel to the implant'srotational symmetry axis of the device such that the pin is not driven,in accordance with one embodiment of the invention;

FIG. 14( b) is a graphical illustration of an implant that includes aspherical magnet and a switching function, the magnet having a recess,with an external magnetic field orientated parallel to the implant'srotational symmetry axis of the device such that the pin is driven, inaccordance with one embodiment of the invention;

FIG. 15( a) is a graphical illustration of a side view of an implantwith a switching function, the implant including a coil and anencapsulated spherical magnet which is recessed at the areas between thetwo magnet's two poles, with an external magnetic field orientedparallel to the implant's symmetry axis;

FIG. 15( b) is a graphical illustration of a top view of an implant witha switching function, the implant including a coil and an encapsulatedspherical magnet which is recessed at the areas between the magnet's twopoles, with an external magnetic field oriented parallel to theimplant's symmetry axis; and

FIG. 15( c) is a graphical illustration of a top view of an implant witha switching function, the implant including a coil and an encapsulatedspherical magnet which is recessed at the areas between the magnet's twopoles, with an external magnetic field oriented perpendicular to theimplant's symmetry axis.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In illustrative embodiments of the invention, a method and device forreducing the effects of strong magnetic fields on an implanted magnet ispresented. FIG. 3 shows an implant 300 for implementing an embodiment ofthe invention. The implant 300 includes a magnet 302 that is held in theimplant 300 in such a way that the magnet 302 can turn into thedirection of an externally applied magnet field. Thus, the magnet 302does not experience any torque as a result of the external magnet field,nor will it become demagnetized.

The magnet 302 may be enclosed in a housing 303. The housing 303 allowsthe implanted magnet of whatever shape to turn, possibly with somerestrictions, into the external magnetic field lines. In variousembodiments of the invention, magnet 302 is hermetically encapsulated soas to prevent corrosion and/or leakage of the material into the body ofthe implant 300. Material used for the housing/encapsulation 303 mayinclude, without limitation, titanium, nonmagnetic stainless steel, orceramic. Housing 303 may be held together by a weld seam 301 afterinsertion of the magnet 302.

As shown in FIG. 3, magnet 302 is a spherical magnet that is free toturn in any direction. To minimize friction between the magnet 302 andthe housing 303, a lubricant can be placed in the small gap between themagnet 302 and the housing 303. Alternatively, or in addition to the useof lubricant, the magnet 302 and/or the housing 303 may be coated by athin layer of material to minimize friction, such as Teflon.

Instead of a spherical magnet 302, magnet 302 may be of a variety ofshapes. For example, magnet 302 may be, without limitation, a cylinderthat can rotate about its axis. The cylindrical magnet carries amagnetization being normal to its axis. This axis can be arrangedhorizontally parallel to the skin in the plane of the implanted coil.This allows the magnet to turn around its axis, so that it may turn itsmagnetic moment either towards the external holding magnet (as istypically the case during normal use) or so as to align with the fieldgenerated, for example, by a high field MRI scanner, whose field linesin a typical examination position run along the patient's axis. In aslightly different arrangement, the axis of the cylinder is stillmounted in the plane of the implanted coil, but at approximately up to45° off the horizontal plane to be able to partially adjust to (lowerfield) MRI-machines which use vertical magnetic fields. The advantage ofa cylindrical magnet is that its aspect ratio (i.e. diameter vs. length)can be chosen such that for a given volume (which is necessary togenerate an adequate holding force) the thickness of the magnet issmaller than that of a spherically shaped magnet.

In accordance with an embodiment of the invention, the implant mayinclude several smaller magnets instead of one magnet, allowing for athinner design of the implant. For example, FIGS. 4 (a) and 4(b) show atop view and a side view, respectively, of an implant 401 that includesthree spherically shaped magnets 402-404 and a coil 406. These magnets402-404 may be arranged some distance apart from each other, thusfacilitating the orientation of magnets 402-404 primarily along thefield lines of an external holding magnet 405, as shown in FIG. 4( b).

As shown in FIGS. 1 and 4( b), both the implant and the external partheld adjacent to the implant may have coils which are positioned acrossfrom each other. A signal, such as a radio frequency (RF) electricalsignal, is inductively coupled to the implant so as to transfer requiredelectrical power and or other signals. The RF signal may induce eddycurrents in the implant magnet and/or the magnet of the external device.This can cause power losses for the implant system. In accordance withvarious embodiments of the invention, these power losses may be reducedby using an electrically conducting material, such as gold, for thecoating and/or encapsulation of the implanted magnet(s) and/or externalmagnet(s). The coating acts as a short circuit for the incident RF wave,resulting in minimal power dissipation.

Another embodiment of the invention for reducing the effects of strongmagnetic fields on an implanted magnet is shown in FIG. 5( a). Theimplanted magnet 501 is partially surrounded by magnetically softmaterial 502 which creates a low reluctance path for magnetic flux. Thiswill partly shield the magnet 501 from the external magnetic field,reducing both the torque and the demagnetization of magnet 501 to alarge extent. The magnetically soft material 502 may form, withoutlimitation, a potlike shaped structure that includes a cavity in whichthe magnet 501 is positioned. The surface of the magnet 501 not coveredby the magnetically soft material is typically orientated, and generatesa magnetic field, normal to the skin. The magnetically soft material 502may be, without limitation, highly permeable Ferrite, an alloy, or aFerrofluid, which contains tiny magnetic particles suspended in fluid.FIG. 5( b) shows an external part 510 held in place by an implant 505,and the approximate magnetic field generated, in accordance with oneembodiment of the invention. Both the implant 505 and the external part510 include a magnet 506 and 511, a magnetically soft material 506 and512, and a coil 507 and 513, respectively.

Referring back to FIG. 1 and as known in the art, magnet 106 may be amagnetically soft material (also known as a keeper). At present, themagnetically soft material used for the keeper has been limited to solidalloys or Ferrite. The magnetically soft material becomes magnetized inthe direction of an external magnetic field and thus avoids any torque.In accordance with one embodiment of the invention, FIG. 6( a) shows animplant 601 that includes a housing 602 containing Ferrofluid, which, asdescribed above, contains tiny magnetic particles suspended in fluid.The housing 602 is sealed to prevent leakage of the Ferrofluid. Similarto a magnetically soft alloy or Ferrite, when a magnetic field isapplied to the Ferrofluid, the Ferrofluid is magnetized in the directionof an external magnetic field. Thus, when placed adjacent an externalpart 604 that includes a magnet 605, the Ferrofluid becomes magnetizedand becomes attracted to the magnet 605, holding the external part 604adjacent the implant 601. FIG. 6( b) shows magnetization of an implant610 that includes Ferrofluid when immersed in an external magnetic fieldof a MRI scanner. Demagnetization or permanent magnetization in adversedirections is not possible because the Ferrofluid does not assume anysizable permanent magnetization. To obtain a magnetic holding force thatis comparable to the case of an implanted permanent magnet, the volumeof both the external magnet 605 and the Ferrofluid may have to becomparatively larger.

In accordance with another embodiment of the invention, a low-reluctancepart may be added to the implant to improve the magnetic flux betweenthe implanted magnet and the external magnet and/or shield magneticfield lines at the implant side facing towards the inner side of thebody (i.e. in medial direction)—minimizing magnetic resistance andimaging artifacts, especially near the medial side of the implant. Forexample, in accordance with one embodiment of the invention, thelow-reluctance part is a cylinder 701 with a spherical calotte made ofmagnetically soft material, as shown in FIG. 7. The cylinder 701 ispositioned inside a hermetic encapsulation 703 enclosing an implanted,spherical permanent magnet 704. Instead of a cylinder 701, thelow-reluctance part may be a cone, or another shape, with the curvatureof the calotte typically being variable and designed to best fit theshape of the permanent magnet 704. The magnetically soft material ismagnetized and attracted by the permanent magnet 704. The sphericalmagnet 704, which can also be variable in shape, may be coated by a thinlayer of material to minimize friction, such as Teflon®. In otherembodiments, a lubricant may be placed in the small gap between thespherical magnet 704 and the cylinder or cone 701 with sphericalcalotte. The cylinder 701 provides a path of low magnetic resistance(reluctance) for the magnetic field lines facing towards the magnet 705of an external device. Additionally, the permanent magnet 704 isattracted to the low-reluctance part 701, preventing small movements(rattling) of the magnet inside its encapsulation 703.

In further illustrative embodiments of the invention, a method anddevice for preventing induction of excessive voltages in the implantreceiver coil/electronics and in the electrode circuit is presented.These excessive voltages may be induced, for example, by RF pulsesduring an MRI examination.

In accordance with one embodiment of the invention, an implant 801having a permanent magnet 805 positioned in a housing 806 includes aswitching function, as shown in FIG. 8( a). Permanent magnet 805 is freeto rotate in response to an external magnetic field, and may be, withoutlimitation, spherical, as shown in FIG. 8( a) or cylindrical in shape(see below). The switching function is achieved through use of alow-reluctance part 802 that includes an electrically conductivesurface, which may be, without limitation, a conductive coating such asaluminum, nickel, copper, gold and/or silver. The low-reluctance part802 is free to move towards or away from the permanent magnet 805 so asto open or close at least one pair of electrical contacts 807-810positioned within the housing 806. Low-reluctance part 802 may vary inshape, including, without limitation, a cylinder with a sphericalcalotte 803 having a curvature designed to best fit the shape ofpermanent magnet 805.

Positioning of low-reluctance part 802, and thus the switching state ofone or more electric contacts 807-810, depends on the presence and theorientation of an external magnetic field, resulting in an attractive orrepulsive force between the permanent magnet 805 and the low-reluctancepart 802. In the absence of an external magnetic field, as shown in FIG.8( a), the low-reluctance part 802 is attracted by the permanent magnet805, as this represents the smallest energy level. In the presence of anexternal magnetic field that is oriented parallel to the implant'srotational symmetry axis 815 as shown in FIG. 8( c), the sphericalmagnet 805 turns to align its magnetic momentum vector with the externalmagnetic field. The low-reluctance part 802 is thus magnetized along itsaxis, generating a magnetic dipole parallel to the symmetry axis 815.This results in an attractive force between the spherical magnet 805 andthe low-reluctance part 802. Alternatively, an external magnetic fieldwhich is oriented normal to the implant's rotational symmetry axis 815,as shown in FIG. 8( b), magnetizes the magnetically soft material 802 ina different direction, turning the spherical magnet 805 into a differentdirection and causing a repulsive force between the spherical magnet 805and the low-reluctance part 802.

Through the use of electrical contacts 807-810, switching functions canthus be exploited. Based on the external magnetic field, thelow-reluctance part 802 is capable of shifting between a first andsecond position, as described above. In the first position, theelectrically conductive surface of the low-reluctance part 802 is insimultaneous contact with a first and a second switching contact 807 and808 (or alternatively 809 and 810), while in the second position, theelectrically conductive surface is out of contact with at least one ofthe first and second switching contacts 807 and 808 (or alternatively809 and 810).

Dependant on configuration, the above-described switching function canbe implemented to close or open electrical circuits based on thepresence of a strong external magnetic field oriented at a certain angleto, for example, the rotational symmetry axis of the device. A switchingfunction which opens an electric circuit during the presence of a strongmagnetic field perpendicular to the symmetry axis of the embodimentcould, for example, be employed to protect receiver circuits of implantsduring MRI. If the rotational symmetry axis of the device is orientedperpendicular to the main magnetic field B₀ in an MR scanner, acomponent of the circularly rotating magnetic RF field B₁ (generated bythe MRI unit) is oriented perpendicular to the receiver coil plane (FIG.8 b), allowing induction of (eventually excessive) voltages which maydestroy the receiver electronics. An opening switch between the receivercoil and the receiver electronic circuit may be used to protect thereceiver electronics. If the rotational symmetry axis of the device isoriented parallel to the main magnetic field B₀ in an MR scanner, theswitch near the magnet remains closed, as shown in FIG. 8 c. Since themagnetic RF field vector B₁ is in this case circularly rotating in thereceiver coil plane, no voltages can be induced in the receiver circuit.Outside the MR scanner, in the presence of external parts of a cochlearimplant system, the external magnet causes a magnetic field in axialdirection, and the magnetic switch is closed. In this mode of operation,the voltages which are generated by the external coil of the implantsystem and which are induced in the implanted coil are passed on to thereceiver electronic circuit. The angle between the orientation of theexternal magnetic field and the rotational symmetry axis of theembodiment at which attraction between the low-reluctance part and themagnet changes into repulsion (i.e. the switching angle) depends on thegeometric dimensions of the low-reluctance part.

Instead of a spherical structure, the permanent magnet 901 may be acylindrical structure which is free to turn around its rotational axis,as shown in FIG. 9, in accordance with one embodiment of the invention.In such embodiments, the magnetically soft part 902 may be a cuboid witha concave side that fits over the magnetic cylinder 901. Both parts 901,902 may be encapsulated and the cuboid 902 may be free to move in avertical direction. A magnetic field orienting in a vertical direction(radially through the cylindrical magnet 901 and vertically through themagnetically soft part 902) would cause an attractive force between thecylindrical magnet 901 and the magnetically soft part 902, whereas amagnetic field orienting in radial direction (horizontally) would causea repulsive force between the two parts 901, 902. A magnetic field in anaxial direction (parallel to the axis of the cylindrical magnet 901)could cause either an attractive or repulsive force, depending on themagnetic properties of the magnetized cylinder 901.

In accordance with another embodiment of the invention, the switchfunction may include a freely turnable permanent magnet 1001 in anon-magnetic housing 1002, wherein a magnetic soft body is not required,as shown in FIG. 10. Permanent magnet 1001 may be, without limitation, aspherical or cylindrical magnet. The permanent magnet 1001 includes atleast one conductive surface 1007 and at least one non-conductivesurface 1003. These surfaces 1002 and 1003 may be implemented by coatingportions of the magnet 1001 with conductive or non-conductive materialswhere appropriate. A first switching contact 1004 and a second switchingcontact 1005 is positioned on the housing 1005. The magnet 1001 iscapable of rotating in response to an external magnetic field to a firstposition, in which the conductive surface is in contact with both thefirst and second contacts 1004 and 1005, and a second position, in whichthe conductive surface is out of contact with at least one of the firstand second contacts 1004 and 1005.

Depending on the orientation of the spherical magnet 1001, which isdefined by the orientation of the external magnetic field, which may begenerated by, without limitation, an external magnet 1006 or an MRscanner, different switching functions (opening or closing of circuits)can be implemented. For example, by conductively coating the sphericalmagnet 1001 over only one magnetic pole, a switching function can beimplemented which opens or closes an electric circuit only at a certainangle and orientation of the external magnetic field. By optimizing thegeometric form of conducting and non-conducting surface areas of thespherical magnet, and/or by adding more switching contacts, a widevariety of even more complex switching functions (e.g. “make beforebreak”) can be defined depending on the orientation of the externalmagnetic field.

In various embodiments, this can be exploited as an over-voltageprotection for an implant receiver circuit in an MR scanner. Inparticular, a switching function between the implant's receiver coil andthe implant's receiver electronic circuit can be implemented so as toprotect the receiver electronics. In accordance with one embodiment ofthe invention, FIG. 11 shows a top view of an implant 1101, such as, butnot limited to a cochlear implant, that includes a spherical magnet 1102encapsulated in a housing 1103 (see FIG. 12( a)). The implant furtherincludes a receiver coil 1104, coupled to a first switching contact 1105and a second switching contact 1106. Referring now to FIG. 12( a), whichis a side view of the implant 1101 of FIG. 11, the coil 1104 ispositioned in a plane and symmetrically coiled around magnet 1102 aboutan axis 1108. The magnet 1102 includes a conductive surface 1107positioned on a South pole of the magnet 1102. If the rotationalsymmetry axis 1108 of the implant 1101 is oriented perpendicular to themain magnetic field B₀ in an MR scanner, a component of the circularlyrotating magnetic RF field B₁ is oriented perpendicular to the receivercoil 1104 plane, allowing possible induction of (eventually excessive)voltages in the coil which could destroy the implant's 1101 electronics.However, the spherical magnet 1102 rotates in response to the externalmagnetic field such that the electrically conductive surface 1107 of themagnet 1102 does not contact either switching contact 1105 and 1106,preventing excessive voltages to be passed to the receiver circuitry.

If the rotational symmetry axis of the implant 1101 is oriented parallelto the main magnetic field B₀ in an MR scanner, the magnet 1102 isorientated such that the electrically conductive surface 1107 of themagnet 1102 contacts both switching contacts 1105 and 1106, and theswitch remains closed, as shown in FIG. 12( b). Since the magnetic RFfield vector B₁ is, in this case, circularly rotating in the receivercoil 1104 plane, no voltages can be induced in the receiver circuit.Outside the MR scanner, in the presence of external parts of a cochlearimplant system, the external magnet causes a magnetic field in the axialdirection, and the magnetic switch is closed. In this mode of operation,the voltages which are generated by the external coil of the implantsystem, and which are induced in the implanted coil 1104, are passed onto the receiver electronic circuit.

A switching function may be also implemented by means other than byelectrical/mechanical switching contacts. For example, switching couldalso depend on different optical properties of the magnet or itssurface.

In accordance with one embodiment of the invention, the switchingfunction is accomplished by a freely turnable permanent magnet 1301 in ahousing 1302, wherein the magnet 1301 is partly recessed and/or grooved,as shown in FIG. 13. By recessing a portion of magnet 1301, such as atone magnetic pole or between magnetic poles, and by coating at least thenon-recessed portion of the magnet 1301 with a conductive material 1303,a switching function depending on the angle and the orientation of theexternal magnetic field from, for example, an external magnet 1306 canbe implemented. In the embodiment shown in FIG. 13, the recessed regionis positioned at the north pole of magnet 1301. Accordingly, switchingcontacts 1304 and 1305 make contact with the conductive coating ofmagnet 1301 and are closed, unless an external magnetic field isoriented parallel to axis 1306 and towards the skin, whereupon magnet1301 is orientated such that the recessed region is positioned adjacentswitching contacts 1304 and 1305. In various embodiments in which therecessed region of the magnet cannot contact the switching contacts, theentire magnet may be coated with conductive material. Similar toabove-described embodiments, magnet may be, without limitation, aspherical or cylindrical magnet. Housing 1302 may hermetically seal themagnet 1301.

As shown in FIGS. 14( a) and 14(b), switching may also be accomplishedby a pin 1401 which, depending on the orientation of the permanentmagnet 1402 and the magnet's geometrical shape at the pin's location,drives a spring contact 1408, in accordance with one embodiment of theinvention. Magnet 1402, which may be encapsulated in a housing 1403 andmay be hermetically sealed, includes at least one recessed region 1405.Recessed region 1404 may be positioned, without limitation, at a pole orbetween poles of magnet 1402. Magnet 1402 is free to move in response toan external magnetic field, which may be generated for example, by anexternal magnet 1440 or MR scanner. As shown in FIG. 14( b), the pin1401 makes contact with, and is driven by, magnet 1402 when the pin 1401is not aligned with the recessed region 1404. Alternatively, when thepin 1401 is aligned with the recessed region 1404, the pin 1401 is notdriven, as shown in FIG. 14( a).

In accordance with another embodiment of the invention, FIGS. 15 (a-c)shows an implant 1500 that includes a permanent magnet 1501 that isrecessed at an area between the magnet's 1501 two magnetic poles (i.e.at the equator), allowing a switching function based on two pins 1502and 1503 which drive two spring contacts 1504 and 1505 at the equator.FIG. 15( a) is a side view of the implant 1506, while FIGS. 15( b) and15(c) show a top view of the implant 1500 when exposed to an externalmagnetic orientated perpendicular and parallel to the skin surface,respectively. Similar to the embodiment shown in FIG. 12, the implant1500 requires no additional space in the axial direction and, therefore,allows a slim implant design. In the presence of an external magneticfield oriented parallel to the rotational symmetry axis of the implant1500 (i.e. perpendicular to the skin), both spring contacts 1504 and1505 are closed. Implant coil 1507 is connected to the implantelectronic circuit, as shown in FIGS. 15( a) and 15(b). Such a magneticfield may be generated, for example, by an external magnet 1520associated with the implant system. When the external magnetic field isoriented perpendicular to the rotational symmetry axis of the embodiment(ie. parallel to the skin), as when using an MR scanner, at least one ofthe two contacts 1504 or 1505 is open, as shown in FIG. 15( c). Theimplant coil 1507 is not connected to the implant electronic circuit.Thus, similar to the embodiment of FIGS. 12( a) and 12 (b), the implant1500 can be exploited to protect the coil 1507 and electronic circuit ofthe implant 1506 during an MRI examination. Note that in order to ensurethat the coil 1507 is connected to the implant electronics only when theexternal magnetic field is oriented parallel to the rotational symmetryaxis (ie. perpendicular to the skin), it is necessary to have twoswitching contacts 1504 and 1505 which are placed in an angle of about90° against each other. If there was only one switching contact (e.g.the closed contact in FIG. 15 c), the coil 1507 would be connected tothe implant electronics when the external magnetic field is oriented asshown in FIG. 15 c.

Like for an implant receiver circuit, a switching-function, as describedabove in accordance with various embodiments of the invention, may alsobe employed to sufficiently protect an electrode circuit of an implantedstimulator against excessive voltage induction during MRI, if theorientation of the electrode loop plane is known. Electrode circuits maybe used, for example, in a cochlear implant to directly stimulate theacoustic nerve.

Additionally, the switching function, as described above in accordancewith various embodiments of the invention, may not only be employed toprevent induction of eventually excessive voltages in the implantreceiver coil/electronics and in the electrode circuit during an MRIexamination, but also for various other reasons which are not limited toMRI examinations. For example, switching functions in an implant couldbe employed to: switch between a “standard operating” mode and“telemetry” mode of an implant; switch into a mode for re-charging of animplanted battery; de-tune a receiver circuit or to adjust for differentreceiver frequencies; and optimize RF coupling or power efficiency of aninductive link system.

Furthermore, applications for the magnetically activated switches, asdescribed above in accordance with various embodiments of the invention,is not restricted to the field of medical implants and may be used forimplementations other than to avoid induction of potential risks (e.g.torque to the implant magnet, weakening of the implant magnet, inductionof excessive voltages in implant circuits) during an MRI examination.The invention could be applied in any area where strong magnetic fieldscan cause (partial) demagnetization of a permanent magnet or where ahazardous torque is exerted to the magnet. Further, the above-describedembodiments of the invention which include electrical switches that aremagnetically activated and where the switching state depends on theorientation of the external magnetic field, could be exploited in anyother electronic device, whereby receiver coils are switched off bydefault and can be activated by applying an external magnetic field of acertain orientation.

Additional magnetically sensitive switching elements which may be usedto enhance MRI safety of implants, include, but are not limited to: Reedcontacts, Hall generators, and Magnetic-field Dependent Resistors(MDR's). The directional sensitivity of these elements may be used withadvantage, since in many applications only particular field directionshave adverse effects. One advantage that the above-describedmagneto-switches of the present invention have over REED switches isthat the magneto-switches have a switching function at a well-definedangle between the external magnetic field and the symmetry axis of thedevice. Another advantage that the magneto-switches of the presentinvention have over Hall generators and MDR's are that they do notrequire a power supply for switching, since they are passive devices.

The described embodiments of the invention are intended to be merelyexemplary and numerous variations and modifications will be apparent tothose skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inthe appended claims.

1. An implant comprising: a housing; and a magnet held by the housing,wherein the magnet is free to turn in any direction without substantialaxial movement.
 2. The implant according to claim 1, wherein the magnetis capable of aligning at least partially with an external magneticfield.
 3. The implant according to claim 1, further including alubricant between the housing the magnet.
 4. The implant according toclaim 1, wherein the magnet is prevented from substantially rattlingagainst the housing.
 5. The implant according to claim 1, wherein theimplant is a cochlear implant.
 6. The implant according to claim 5,wherein the magnet is a spherical magnet.
 7. The implant according toclaim 1, further comprising a first coil adapted to beelectromagnetically coupled with a second coil of an external device. 8.The implant according to claim 1, wherein the housing is anencapsulation that hermetically seals the magnet.
 9. The implantaccording to claim 1, wherein the housing includes a magnetically softmaterial at least partially surrounding the magnet.
 10. The implantaccording to claim 1, wherein the housing includes a material selectedfrom the group of materials consisting of titanium, nonmagneticstainless steel, and ceramic.
 11. The implant according to claim 1,wherein one of the housing and the magnet includes a Teflon surface. 12.The implant according to claim 1, wherein the magnet is coated with aconductive surface.
 13. The implant according to claim 1, wherein themagnet is spherically shaped.
 14. An implant comprising: a magnet:holding means for holding the magnet such that the magnet is free toturn in any direction without substantial axial movement, wherein themagnet is capable of aligning at least partially with an externalmagnetic field.
 15. The implant according to claim 14, wherein theholding means prevents the magnet from substantially rattling.
 16. Theimplant according to claim 14, wherein the implant is a cochlearimplant.
 17. The implant according to claim 16, wherein the magnet is aspherical magnet.
 18. The implant according to claim 14, furthercomprising a first coil adapted to be electromagnetically coupled with asecond coil of an external device.
 19. The implant according to claim14, wherein the holding means includes a magnetically soft material atleast partially surrounding the magnet.
 20. The implant according toclaim 14, wherein one of the holding means and the magnet includes aTeflon surface.